1. Field of the Invention
The present invention relates to a convergence calibration method, and more particularly, to a convergence calibration method for a display using multiple beams to generate a video display on a screen.
2. Description of the Prior Art
Increasing demand for large screen televisions is giving rise to an expanding market for planar televisions. Advanced technologies overcome the obstacles of manufacturing large-scale planar displays and gradually lower the prices of the products. Therefore, large-scale planar displays, once appearing as expensive professional equipment, have found an increasing number of household applications, such as in home theaters. Various projection techniques aiming at providing better visual experience are constantly being researched and developed. Among them, rear projection displays feature high contrast, high brightness, large viewing angles, sharp picture definition and low geometrical distortion, and are therefore very competitive in the market.
A typical rear projection color television set includes three cathode ray tubes (CRTs), each CRT processing one of the primary colors: red, blue or green. By combining the three monochromatic beams, the set can produce full color television pictures. Please refer to FIG. 1 for a plan view of a rear projection television set 10. The rear projection television set 10 shown in FIG. 1 includes a red CRT 12, a lens 22, a mirror 18, a base module 19, and a screen 20. The rear projection television set 10 further includes another two CRTs and corresponding lenses for each CRT, although for clarity, only a single CRT (the red CRT 12) and a single lens (the lens 22) are shown in FIG. 1. The light from the red CRT 12 passes through the lens 22 and illuminates the mirror 18, which in turn reflects the light onto the screen 20 for observation by the viewer.
Please refer to FIG. 2 illustrating the relationship between the three CRTs of the rear projection television set 10. The rear projection television set 10 include the red CRT 12, a green CRT 14, a blue CRT 16 and lenses 22, 24, 26. As can be seen in the figure, CRTs 12, 14 and 16 are matched respectively with lenses 22, 24 and 26. The rear projection television set 10 sends a red color signal R, a green color signal G and a blue color signal B of a display image to the red CRT 12, the green CRT 14, and the blue CRT 16, respectively. Color signals R, G, B are enlarged by the lenses 22, 24, 26 respectively. The enlarged color signals then illuminate the mirror 18, which in turn reflects the light onto the screen 20. By combining the three monochromatic beams of the color signals R, G, B, the rear projection television set 10 can produce full color images on the screen 20. In order to produce accurate pictures, proper alignment of the beams must be maintained. Based on the sizes of the base module 19 and the screen 20, the CRTs 12, 14, 16 and the lenses 22, 24, 26 are disposed at a certain angle with respect to the screen 20 so that their beams are focused at the same point on the screen.
However, the magnitude of the earth's magnetic field at the position where the rear projection television set 10 is installed and used is different from that of the earth's magnetic field at the position of the manufacturing production line, since the region and direction are different from each other. Accordingly, the change of the magnitude of the earth's magnetic field affects the precise forming of the picture of the rear projection television set 10. In other words, the change of the earth's magnetic field causes a distortion in the deflection degree of the cathode ray, thus the exact picture cannot be formed on the screen 20. Therefore, the rear projection television set 10 has a built-in convergence control circuit for compensating for the influence of the change of the earth's magnetic field.
Please refer to FIG. 3 for a prior art convergence calibration circuit 30 and FIG. 4 for a diagram illustrating a prior art method of performing convergence calibration by the convergence calibration circuit 30 disclosed in U.S. Pat. No. 6,330,040 to Kawashima, which is included herein by reference. In FIG. 3 sensors 64, 66, 68 and 70 are disposed around the screen 20, onto which test patterns are projected by a pattern generator 100 and CRTs 82, 84 and 86. An I/V converter 72 convert current signals measured by the sensors 64, 66, 68 and 70 from the test patterns into voltage signals, which in turn pass through a low-pass filter (LPF) 73 and are converted into digital signals by an analog-to-digital converter (ADC) 74. A controller 76 performs calculations for the convergence calibration and sends adjustment signals to a diffraction wave generator 78. Based on the received adjustment signals, the diffraction wave generator 78 generates driving signals for a yoke driver 80. The yoke driver 80 controls the CRTs 82, 84 and 86 until a correct adjustment value for the convergence calibration is acquired. As shown in FIG. 4, “O” and “X” represent the center of the screen 20 and the center of a testing matrix, respectively. When the CRTs 82, 84 and 86 project test patterns based on the correct adjustment value for the convergence calibration, “O” and “X” will be completely overlapped. Since the sensor output has a non-linear relationship with respect to the light wavelength, it also influences the accuracy of the prior art method shown in FIG. 3 and FIG. 4. Also, many peripheral circuits are required, making the convergence calibration very complicated.
Please refer to FIG. 5 for a prior art method of performing convergence calibration disclosed in US patent publication No. 20030030757 to Shim , which is included herein by reference. In FIG. 5, four optical sensors T1-T4 are disposed at an upper side, a left side, a lower side, and a right side of the screen 20, respectively, and predetermined reference patterns PH and PV are displayed on the screen 20. The reference patterns consist of a horizontal reference pattern PH displayed as a horizontal line, and a vertical reference pattern PV displayed as a vertical line. The horizontal reference pattern PH moves from an upper part of the screen 20 towards a lower part of the screen 20 as indicated by a vertical arrow, and the vertical reference pattern PV moves from a left part of the screen 20 to a right part of the screen 20 as indicated by a horizontal arrow. The optical sensors T1-T4 measure a luminance of the reference patterns PV and PH moving horizontally and vertically, respectively. Convergence calibration is then performed based on data measured by the sensors.
In the prior art method of performing convergence calibration illustrated in FIG. 3-5, the sensors measure luminance of the reference patterns when the reference patterns move across the sensors. The prior art method is largely influenced by background noises that interfere with actual signals of reference patterns. Therefore, the accuracy of convergence calibration is also affected. And since the sensor output has a non-linear relationship with respect to the light wavelength, the convergence calculation and adjustment are also influenced.